Pressure release lock



June 2, 1959 G. s. ORMSBY ETAL PRESSURE RELEASE LOCK 3 Sheets-Sheet 1 Filed Oct. 18, 1954 INVENTORS G.S.ORMSBY HM M g I 'F/G.

A 7' TORNE KS June 2, 1959 c. s. ORMSBY ETAL 2,883,990

PRESSURE RELEASE LOCK Filed Oct. 18. 1954 i Y s Sheets-Sheet 2 i 24 5 I I I 56 b INVENTORS R. A. 8050 .J BY 6. s. ORMSBY HLwawl H A T TORNE rs J1me 1959 G. SrORMSBY ETAL PRESSURE RELEASE LOCK 5 Sheets-Sheet 3 Filed 001:. 18, 1954 FIG. 8.

A 7' TORNEYS United States Patent PRESSURE RELEASE LOCK George S. Ormsby and Roy A. Bobo, Houston, Tex., as-

signors toPhillips Petroleum Company, a corporation of Delaware Application October 18, 1954, Serial No. 462,873

19 Claims. (Cl. 166-225) This invention relates to a pressure release lock for use with apparatus in a bore hole. In one specific aspect, it relates to a pressure release lock for a rotary inertia operated check valve.

In completing an oil or gas Well, it is common practice to install a float collar at or near the bottom of the easing string. Such a collar contains a check valve which permits downward flow of fluid through the casing string, but prevents upward flow into the casing. With the use of such a check valve, it is necessary to interrupt the casing running process periodically to fill the casing from the surface with drilling fluid to prevent the casing from collapsing due to the external well pressure exerted thereon. This operation results in a loss of time and may result in the casing becoming stuck in the well bore if the casing is held stationary an excessive time during the filling process. A check valve of this type has further disadvantages. For example, a large pressure may build up opposite the lower end of the casing due to the displaced drilling fluid being forced into the narrow annulus between the casing and the wall of the Well. This may cause a breakdown of a permeable section of the earth formation which results in the loss of drilling fluid and increases the danger of a blow-out and reduces the chances of obtaining the desired fill up with cement.

In our copending application Serial No. 362,265, filed June 17, 1953, there is disclosed an improved form of check valve and casing orifice for use at the lower end of the well casing. The orifice substantially restricts the flow passage into the casing, and the check valve is of such configuration as to be closed at a preselected depth merely by rotating the casing. While this arrangement overcomes the disadvantages of the float check valves of the prior art, there is some danger of the valve closing accidentally before the selected well depth is reached. In accordance with the present invention, this danger is eliminated by a pressure operated collapsible locking means which prevents the check valve from being closed until the apparatus is lowered into the well to a preselected depth. The locking means can be in the form of a retaining pin which is connected to either a pressure charged bellows or a spring biased piston. The bellows or piston is adjusted initially such that the locking pin is removed when the well pressure reaches a predetermined value.

Accordingly, it is an object of this invention to provide a pressure release lock for use with apparatus operated within a well.

Another object is to provide a pressure release lock for a rotary inertia operated check valve.

' Figure 1 is an elevation view, shown partially in section, of a first embodiment of this invention;

Figure 2 is a sectional view taken along line 2--2 in Figure 1;

Figure 3 is a sectional view taken along line 33 in Figure 1;

Figure 4 is a detailed view, shown partially in section, of the pressure actuated locking mechanism of Figure 1;

Figure 5 is a perspective view of a second embodiment of the check valve release mechanism;

Figure 6 is a detailed view, shown partially in section,

of a third embodiment of the ball release mechanism;

Figure 7 is a sectional view taken along line 6--6 in Figure 6;

Figure 8 is a longitudinal view, shown partially in section, of a fourth embodiment of the invention;

Figure 9 is a sectional view taken along line 9-9 int Figure 8; and

Figure 10 is a detailed view, shown partially in section,. of a second embodiment of the pressure actuated release pm.

Referring now to the drawing in detail and to Figures: 1,2, 3 and 4 in particular, there is shown the lower end of a casing string 10. A casing shoe or collar 11 which forms a housing is suspended from casing 10 by a coupling 12. An annular valve seat member 13 is positioned within shoe 11 near the upper end thereof. A central passage 14 is formed in seat member 13, the upper end of passage 14 being of smaller diameter than the lower end. The junction between the two sections of passage 14 is beveled to form a seat 15 for an adaptor plug 16. This beveled section contains a sealing ring 17. The lower end of adaptor plug 16 is attached to a plurality of fins 19 which extend inwardly from the wall of shoe 11.

An annular orifice member 21 is threaded inside adapter 16, and a sealing ring 22 is disposed between the two elements. The lower end of orifice member 21 is beveled to form a valve seat 23 which is adapted to receive a ball 24, thereby forming a check valve. An annular spring stop 26 is attached to the wall of shoe 11 in spaced relation with the lower ends of fins 19. A compression spring 27 is interposed between stop ring 26 and fins 19 to retain the orifice assembly in the illustrated position in engagement with annular member 13'. Spring 27 normally retains the orifice assembly in this position. However, the flow of drilling fluid downwardly through casing 10 may exert suflicient force on member 21 to compress spring 27, thereby enlarging the passage through annular member 13.

Since there is a tendency for drilling fluid to flow upwardly through casing 10 at the time the casing is lowered into a well, the insertable orifice assembly is useful in restricting the flow passage through member 13. This causes a portion of the drilling fluid to rise in the am nular space between casing 10 and the wall of the well so i I that any excess of drilling fluid overflows into the mud A further object is to provide a pressure release lock pit at the surface rather than onto the drilling platform. The inner diameter of orifice member 21 is selected so that the drilling fluid is retained at a desired depth within the well.

Once the casing has been lowered into the well and it is desired to cement the casing to the well wall, cement is pumped into the top of the casing in a conventional manner. When the required amount of cement has been added, a plug is inserted into the casing and forced downwardly by pumping additional drilling fluid in the casing. When this plug reaches annular member 13,. the cement has been forced through the apparatus into the well and additional flow is blocked. Ball 24 is then released from the position shown inside cage 30 to block p the passage through .orifice member 21, thereby preventing fluid flow upwardly into casing 10. Ball 24 can be 7 Patented June 2, 1959 released after casing has reached the desired depth, but before the cement is added. In some instances it may be desirable to release ball 24 before casing 10 reaches the desired depth to prevent blowout through the casing. Ball 24 initially is retained in a funnel shaped housing 30. A retaining pin 31 is fitted in the large diameter end of housing 30 to prevent ball 24 from escaping therethrough. One or more spring loaded retaining pins 32 is provided in the small diameter end of housing 30' to prevent ball 24 from rolling out of housing 30 inadvertently. In addition, a second retaining pin 34 is positioned across the small diameter end of housing 31) by a guide 35 and a bellows 36, the latter being attached to a support member 37. The interior of bellows 36 initially is filled through a plug 38 with a gas at a preselected pressure. This pressure results in the bellows being expanded such that pin 34 blocks the outlet passage in housing 30. However, as the device is lowered into the well, the hydrostatic pressure exerted on bellows 36 tends to overcome the force of the gas pressure within the bellows such that the bellows is collapsed gradually and pin 34 is withdrawn from housing 30. The initial pressure in bellows 36 is adjusted such that pin 34 is withdrawn completely when the device is lowered to a preselected depth. When the first few lengths of casing are assembled near the surface of the well, there is some danger of ball 24 accidentally being rotated out of housing 30. For this reason, the bellows and pin assembly is provided to prevent such movement of ball 24. Pin 34 is set to be withdrawn before the depth is reached where it is desired to release ball 24.

When it is desired to place the check valve in operation by the release of ball 24, the well casing is rotated in a clockwise direction, looking downwardly. This rotation tends to cause a relatively large volume of the well fluid to flow into the large diameter end of housing 30 to force ball 24 out through the small diameter opening. The force of this Well fluid is sufficient to overcome the retaining force of pins 32. Ball 24 is constructed to have a specific gravity less than the specific gravity of the well fluid. Thus, the ball rises until seated on valve seat 23, thereby blocking the passage through orifice member 21. A grating 39 is provided in the lower portion of shoe 11 to prevent ball 24 from falling out of the shoe in the event there is a downward flow of fluid through casing 10.

It is preferred that the apparatus contained within casing 11 be formed of a material which can be removed by a drill bit in the event it becomes necessary to drill to a deeper depth after the casing has been set. The apparatus is preferably constructed of a plastic, resinous or polymeric material, or even of light metals which can readily be removed by a drill bit. The material selected should be sufliciently strong to withstand the pressures encountered in the well bore, but yet should be of such a nature that it can readily be ground up and then removed from the Well by normal drilling fluid circulation.

In Figure 5, there is shown a second embodiment of the ball release mechanism. Ball 24 is contained in a cage 40 which is secured to the wall of easing shoe 11. The inner edge of cage 40 is secured to a stationary shaft 41 which is attached to shoe 11 at its upper end by a spider assembly 42 and at its lower end by a spider assembly 43. Assembly 42 is of such configuration that ball 24 can readily pass therethrough while spider 43 prevents ball 24 from falling therebeneath. A stop member 44 is positioned across the first opening of cage 40 to prevent ball 24 from rolling out of this opening. One or more spring loaded retaining pins 32 and a pressure responsive pin 34 are positioned across the second open ing of cage 40. A paddle arm or baffle 46 having a relathe assembly is balanced dynamically. In this manner, paddle arm 46 and sweep arm 48 are free to rotate about shaft 41. The side of the sweep arm adjacent stop member 44 has a plane surface whereas the opposite side of the sweep arm is provided with a convex surface. Thus, when fluid flows either upwardly or downwardly through shoe 11, sweep arm 48 tends to rotate in a clockwise direction, looking downwardly, about shaft 41 and thus remains out of engagement with ball 24 in cage 40.

. shoe 11.

When it is desired to close the ball operated check valve, the well casing is rotated in a clockwise direction so that cage 40 and ball 24 are likewise rotated in a clockwise direction. However, the assembly of paddle arm 46 and sweep arm 48 tends to remain stationary because of the combined inertia of the drilling fluid and the assembly. Continued rotation of the casing thus displaces cage 46 so that sweep arm 48 enters the cage beneath retaining plate 44 and forces ball 24 out of the cage by overcoming the force of retaining pin 32. It is assumed that the device has been lowered into the well a sufiicient depth such that pin 34 is removed in the manner described above in conjunction with Figure 1. Ball 24 then rises into engagement with valve seat 23 of Figure 1.

In Figures 6 and 7, there is shown a third embodiment of the check valve ball release mechanism. Ball 24 is disposed within a cage 55 which is secured to the wall of An inertia paddle 56 is mounted for rotation about a vertical shaft 41 which corresponds to shaft 41 of Figure 5. A pressure actuated lock pin 34 is disposed across one opening of cage 55 to retain ball 24 within the cage until the mechanism is lowered to the preselected depth. A hinge plate 57 is retained by a compression spring 58 in engagement with ball 24 adjacent pin 34 so as to retain the ball within the cage even after pin 34 in removed. A plunger 59 extends from paddle 56 into the second opening of cage 55. A stop member 60 is secured to shoe 11 to prevent paddle 56 from rotating in a counterclockwise direction, see Figure 6. When it is desired to release ball 24 from cage 55, the casing is rotated in a clockwise direction for a short time and then is stopped suddenly. The inertia of paddle 56 causes this member and plunger 59 to continue to rotate in a clockwise direction, thereby ejecting ball 24 from cage 55 against the force of retaining spring 58. Ball 24 then rises into engagement with valve seat 23, as shown in Figure 1.

In Figure 8,. there is show a fourth embodiment of a rotary inertia operated check valve assembly. In the lower portion of shoe 11, there is provided a pair of rings 63 and 64 which supports a spider assembly 65. A bearing assembly 66 is mounted on spider assembly to support a cam shaft 67 having an opening 73 in the upper end thereof. The upper end of cam shaft 67 is supported by a bearing assembly 68 which is retained in position by a spider assembly 69. Bearing assembly 68 has an opening or slot 70 formed in one side thereof. Within the opening 70 and attached to bearing 68 is a more or less rounded element 71 to serve as a guide for the yoke end of a cam rod 72. The cam rod 72 is intended to tively large surface is provided in close proximity to the loosely about shaft 41. A sweep arm 48 is attached to collar 47 on the opposite side rom paddle arm 46 so that touch the circular surface of cam shaft 67 at a point near its top. When the shaft 67 is in the position shown, the cam rod 72 is at the left extremity of its movement. In the position illustrated, opening 73 is 180 away from the yoke end of cam rod 72. The second end of cam rod 72 engages the lower end of a plug 74, which serves as a valve, which is attached to shoe 11 at a spring biased pivot 75. Guide member 76 retains rod 72 in engagement with the lower end of plug 74 to maintain plug 74 v in the position shown by the solid lines.

An annular member 77 is positioned in the upper end of shoe 11 to form a passage 78. A flapper choke or orifice member assembly is hinged at pivot 81 in a manner so as to be free to rotate upwardly into passage 78 to form a smaller diameter orifice and a valve seat for plug 74. Orifice member 80 normally is in engageassaeee intent with member 7'7 when fluid is flowing upwardly through shoe 11, but isdisplaced downwardly when fluid flows in the reverse direction.

Before the apparatus is lowered into the well, cam shaft 67 is rotated to the illustrated position so that re-. cess 73 is out of engagement with cam rod 72. The cam shaft is retained in this position by means of a paddle arm 83 which is secured thereto. Arm 83 is retained in a fixed position by a stationary stop member 84 on one side thereof and a pressure actuated pin 34 on the opposite side thereof. A guide arm 85, which corresponds generally to sweep arm 48 of Figure 5, is attached to cam shaft 67 on the side opposite paddle arm 83. The guide arm 85 thus retains paddle arm 83 in the illustrated position even after the retaining force of pin 34 is removed. When it is desired to seat the check valve, the casing string is rotated in a clockwise direction, looking downwardly, which results in paddle arm 83 remaining stationary. Cam shaft 67 is thus rotated until opening 73 is adjacent rod 72. Rod 72 is then pushed to the right by spring loaded plug 74 into opening 73 and plug 74 swings upwardly into orifice member 80. The upward force exerted by fluid in shoe 11 and the spring biased pivot 75 retain plug 74 in the seated position.

i In Figure there is shown a second embodiment of the safety pin mechanism which can be employed in any of the illustrated configurations in place of the bellows. Pin 34' is secured to a piston 100 which is contained within a chamber 101. Chamber 101 is secured to a support 37. The interior of chamber 101 initially is supplied with pneumatic pressure through plug 106 to move piston 100 to the illustrated position. A guard ring 103 prevents the piston from being expelled from chamher 101. The hydrostatic well pressure tends to overcome this pneumatic pressure to move pin 34 into chamber 101.

From the foregoing description of preferred embodiments of this invention, it should be apparent that there is provided an improved rotary inertia check valve incorporating a pressure responsive release lock. This release lock prevents the check valve from being operated inadvertently before the assembly is lowered into the well to a preselected depth. This device prevents accidental operation of the check valve which might be caused by rotation or jarring of the well casing at or near the surface. The device can be set so that the safety release pin is removed at a relatively shallow depth, whereby the check valve can be operated if there is danger of a blowout. While the apparatus of this invention has been described in conjunction with present preferred embodiments, it should be apparent that the invention is not limited thereto.

What is claimed is:

1. In a valve assembly comprising, in combination, a housing adapted to be suspended in a well bore by a conduit, a valve member disposed within said housing and adapted in its operative position to prevent upward fluid flow through said housing, and mass inertia valve releasing means disposed within said housing and adapted to allow said valve member to move to its said operative position upon rotation of said housing, the improvement comprising pressure responsive locking means disposed within said housing and adapted to preliminary retain said valve member in an inoperative position until said locking means is subjected to a predetermined hydrostatic pressure.

12. In a valve assembly comprising, in combination, a

tubular member adapted to form part of a conduit string and suspended by the latter in a well bore, a valve seat Within said tubular member, flow control means providing a restricted passage in said tubular member through which fluid can flow upwardly, a valve member disposed insaid tubular member and adapted in its operative position to engage said valve seat and close said restricted assage to prevent said upward flow of fluid, releasable holding means adapted to hold said valve member in art sure responsive locking means disposed within said tubular,

member and adapted to preliminarily retain said valve member in an inoperative position until said locking means is subjected to a predetermined hydrostatic pressure.

3. The combination according to claim 2 wherein said pressure responsive locking means comprises a collapsible member rigidly secured at one end and a pin secured to the other end thereof, said pin movable with said collapsible member, said pin being displaced when said collapsible member is subjected to said predetermined hydrostatic pressure.

4. In a ball check valve assembly comprising, in combination, a tubular member adapted to form part of a conduit string and suspended by the latter in a well bore, a valve seat within said tubular member, flow control means providing a restricted passage in said tubular member through which fluid can flow upwardly, a cage secured within said tubular member below said valve seat, a ball disposed within said cage, said ball when removed from said cage adapted in its operative position to engage said valve seat and close said restricted passage to prevent said upward flow of fluid, releasable holding means adapted to hold said ball in said cage, and mass inertia valve releasing means disposed within said tubular member and adapted to release said holding means to allow said ball to be removed from said cage and engage said valve seat upon rotation of said tubular memher, the improvement comprising pressure responsive looking means disposed within said tubular member and adapted to preliminarily retain said ball within. said cage until said locking means is subjected to a predetermined hydrostatic pressure.

5. The combination in accordance with claim 4 wherein said pressure responsive locking means comprises a first member disposed to block the exit from said cage, 2. collapsible member, means securing one end of said collapsible member to said tubular member, and means securing the second end of said collapsible member to said first member whereby said first member is removed from the exit of said cage, said predetermined hydrostatic pressure is applied to said collapsible member.

6. The combination in accordance with claim 5 wherein said collapsible member comprises a bellows having the interior thereof filled with a gas at a preselected pressure. 7. The combination in accordance with claim 5 wherein said collapsible member comprises a piston chamber, a

piston disposed therein, said piston chamber being supplied with pneumatic pressure to urge said piston to one end of said chamber, the side of said piston opposite said pneumatic pressure being exposed to said predetermined hydrostatic pressure in said tubular member, said piston and said piston chamber comprising the two ends of said collapsible member.

8. In a check valve assembly comprising, in combination, a housing adapted to be suspended in a well bore a releasing means, the improvement comprising pressure responsive locking means disposed within said housing and adapted to preliminarily retain said valve member in an inoperative position until said locking means is subjected to a predetermined hydrostatic pressure.

9. In a check valve assembly comprising, in combination, a tubular member adapted to form part of a conduit string and suspended by the latter in a Well bore, a valve seat within said tubular member, pivotal flow control means providing a restricted passage in said tubular member through which fluid can flow upwardly, a spring loaded hinged valve member within said tubular member below said valve seat and adapted in its operative position to engage said valve seat and close said restricted passage to prevent upward flow of fluid, releasable holding means adapted to hold said valve member in an inoperative position, said valve member and said holding means being rotatable with said tubular member, stationary means within said tubular member, and a mass inertia member secured to said stationary means and adapted to hold the same relatively stationary with respect to said tubular member when the latter is rotated, said stationary means adapted to release said holding means to allow said valve member to move to its said operative position upon rotation of said tubular member with respect to said mass inertia valve releasing means, the improvement comprising pressure responsive locking means disposed within said tubular member and adapted to preliminarily retain said valve member in an inoperative position until said locking means is subjected to a predetermined hydrostatic pressure.

10. In a check valve assembly comprising, in combination, a tubular member adapted to form part of a conduit string and suspended by the latter in a well bore, a valve seat within said tubular member, a hinged orifice member adapted to engage said seat and providing a restricted passage in said tubular member through which fluid can flow upwardly, a spring loaded hinged valve member within said tubular member adapted to bias said orifice member against said seat and adapted in its operative position to close said restricted passage to prevent upward flow of fluid, said orifice member and valve member being deflectable to inoperative position by downward flow of fluid, a releasable cam rod having one end adapted to hold said valve member in its inoperative position, said orifice member, valve member, and cam rod being rotatable with said tubular housing, stationary cam shaft axially disposed'in said tubular member, the other end of said cam rod adapted to engage the cam of said cam shaft when said cam is in its operative position so as to release said cam rod to allow said valve member to move to its operative position, a mass inertia member secured to said cam shaft and adapted to hold the same stationary with respect to said tubular member when the latter is rotated whereby said cam in its operative position engages said cam rod, and stop means secured to said tubular member and adapted to engage said mass inertia member to cause it to remain in a fixed position, the improvement comprising pressure responsive locking means disposed within said tubular member and adapted to retain said mass inertia member rotatable with said tubular member until said locking means is subjected to a predetermined hydrostatic pressure.

. 11. In a ball check valve assembly comprising, in combination, a tubular member adapted to form part of a conduit string and suspended by the latter in a well bore, an annular collar attached to the upper inner wall of said tubular member, an annular orifice member longitudinally moveable within said annular collar and provided with a restricted passage through which fluid can flow upwardly, a valve seat defined by said orifice member, spring biasing means urging said orifice member into engagement with said collar member, a generally funnel-shaped cage secured within said tubular member, said cage being open at both ends for fluid communication therebetween, a ball positioned Within said cage intermediate said open ends,

stop means, positioned across the large open end of said cage to prevent the exit of said ball fromtsaid large open open end of said cage to retain said ball within said cage until said tubular member is rotated in a direction and at suflicient speed for fluid to enter said large open end of said cage to force said ball out of said small open end of said cage against the force of said resilient holding means whereby said ball is allowed to move to an operative position to engage said valve seat to prevent upward flow of fluid through said restricted passage, the improvement comprising pressure responsive locking means secured to said tubular member to retain said ball within said cage until said locking means is subjected to a prebination, a tubular member adapted to form part of a conduit string and suspended by the later in a well bore, an annular collar attached to the upper inner wall of said tubular member, an annular orifice member logitudinally moveable within said annular collar and provided with a restricted passage through which fluid can flow upwardly, a valve seat defined by said orifice member, spring biasing means urging said orifice member into engagement with said collar member, a cage secured to said tubular member and open at both ends to permit fluid communication therebetween, a ball positioned within said cage intermediate said open ends, stop means positioned across one of said open ends to prevent the exit of said ball from said one end, releasable holding means positioned across the other of said open ends to retain said ball within said cage, inertia means comprising a paddle arm and a sweep arm secured together, said inertia means suspended freely within said tubular member whereby said sweep arm enters said cage to expel said ball through said other open end against the force of said resilient holding means when said tubular member is rotated, the improvement comprising pressure responsive locking means secured to said tubular member to retain said ball within said cage until said locking means is subjected to a predetermined hydrostatic pressure.

14. The combination in accordance with claim 13 wherein said pressure responsive locking means comprises a first member disposed to block the exit from said cage,

a collapsible member, means securing one end of said collapsible member to said tubular member, and means securing the second end of said collapsible member 'to said first member whereby said first member is removed from the exit of said cage when said predetermined hydrostatic pressure is applied to said collapsible member.

15. In a ball check valve assembly comprising, in combination, a tubular member adapted to form part of a conduit string and suspended by the latter in a well bore, an annular collar attached to the upper inner wall of said tubular member, an annular orifice member longitudinalw ly movable within said annular collar and provided with a restricted passage through which fluid can flow upwardly, a valve seat defined by said orifice member, spring biasing means urging said orifice member intoengagement with said collar member, a cage secured to said tubular member and open at both ends to permit fluid communication therebetween, a ball positioned within said cage intermediate said open ends, resilient holding means positioned-across one of said open ends of said cage to retainsaid ball within said cage, inertia means comprising a paddle arm and an extension element, said extension member normally blocking the other of said open ends, said inertia means being freely suspended within 9 said housing whereby said extension element enters said other open end of said cage to expel said ball through said one end of said cage against the force of said resilient means when said tubular member is rotated, the improvement comprising pressure responsive locking means secured to said tubular member to retain said ball within said cage until said locking means is subjected to a predetermined hydrostatic pressure.

16. The combination in accordance with claim 15 wherein said pressure responsive locking means comprises a first member disposed to block the exit from said cage, a collapsible member, means securing one end of said collapsible member to said tubular member, and means securing the second end of said collapsible member to said first member whereby said first member is removed from the exit of said cage when said predetermined hydrostatic pressure is applied to said collapsible member.

17. A pressure release lock comprising a collapsible member, means positioning one end of said member rigidly, a pin secured to the second end of said member and 10 movable therewith whereby the displacement of said pin is a function of the external pressure on said member.

18. The combination in accordance with claim 17 wherein said collapsible member comprises a piston chamber, a piston disposed therein, said piston chamber being supplied with pneumatic pressure to urge said piston to one end of said chamber, the side of said piston opposite said pneumatic pressure being exposed to the pressure in said housing, said piston and said piston chamber comprising the two ends of said collapsible member.

19. The combination in accordance with claim 17 wherein said collapsible member comprises a bellows having the interior thereof filled with a gas at a preselected pressure.

References Cited in the file of this patent UNITED STATES PATENTS 

